Re: ERROR 2013 (HY000): Lost connection to MySQL server during query.

[Peter Brawley]

This is on a master in a replication setup?

If it's InnoDB with 8.0 see https://mysqlserverteam.com/mysql-8-0-innodb-now-supports-instant-add-column/.

Otherwise try ...

1 Record a starting time or replication log position

2 Disable transactions

3 Make a working copy of the table, add the desired column to it

4 Give the original table a backup name

5 Rename the new table to the original name

6 Replay the transaction log from the timestamp in #1.

7 Enable transactions

If you need more help, tell us the MySQL version and show result of Show Create Table for the table.